The invention concerns an apparatus and a method for wavelength detection with fiber Bragg grating sensors (referred to hereafter as FBG sensors) and in particular a wavelength detector configuration for FBG sensors that are adapted to dynamic stretching or compression measurements (referred to hereafter as strain measurements).
Among the numerous applications of fiber-optic sensors (cf A. D. Kersey in xe2x80x9cOptical Fiber Technologyxe2x80x9d, volume 2, 1996, p 291 ff), fiber sensors with integrated Bragg gratings take on special significance because of their general suitability for strain or temperature measurements. Each Bragg grating in a FBG sensor reflects light with a characteristic Bragg wavelength that is dependent on parameters of the grating integrated into the fiber and thus on changes in the length of the fiber, eg through compression, strain or temperature effects. FBG sensors are used in construction engineering, for example, to detect structural stress.
It is known that a plurality of Bragg gratings with different characteristic Bragg wavelengths can be arranged in an optical fiber and sampled selectively by wavelength to enable a locally resolved measurement of stress, eg in a building structure. For stress measurement, a broadband light source radiates into the FBG sensor and the light, dependent on strain, reflected by each grating is detected. Depending on the detector system, the wavelengths of the reflected component signals are measured and thus the current strain parameter at the location of the particular grating is determined. Dynamic strain measurements require wavelength resolution of at least 50 picometers (pm) at frequencies up to 5000 Hz.
The detection or sampling schematics used to date for determining Bragg wavelengths were summarized by Y.-J. Rao in xe2x80x9cMeas. Sci. Technol.xe2x80x9d, volume 8, 1997, p 355 ff. It is possible, for example, to detect Bragg wavelengths by interferometry (Fabry-Perot interferometer) or by dispersion (continuously tunable prism or grating spectrometers). These methods are unsuitable for dynamic strain measurements at the mentioned high frequencies. Furthermore, dispersive configurations are elaborate and not suited to simultaneously achieving the required wavelength and time domain resolution.
The method of socalled laser tracking does not use a broadband light source. Instead a laser of continuously tunable emission wavelength radiates into the FBG sensor and the reflection spectrum is cyclically sampled. This method presents disadvantages when using several Bragg gratings in an FBG sensor, because a relatively wide range of wavelengths must be sampled, which is difficult at the frequencies necessary for the dynamic measurement.
It is therefore an object of this invention to propose an improved wavelength detector configuration for determining Bragg grating reflection wavelengths that is simple in structure and suitable for dynamic measurements at high frequencies.
It is a further object of this invention to propose a simple extension to form a system for simultaneous detection of the Bragg wavelengths of a large number of Bragg gratings.
It is a further object of this invention to propose a method for application of such detector configurations.
It is a further object of this invention to propose a strain measurement configuration using such an improved wavelength detector configuration detector arrangement.
It is a further object of this invention to propose a method for operating said strain measurement configuration using such an improved wavelength detector configuration detector arrangement.
Other objects of the invention will become apparent to one of ordinary skill in the art from the following description.
The fundamental idea of the invention consists in providing, for each Bragg grating of an FBG sensor, two detectors with narrowband spectral sensitivity ranges that overlap, whereby the Bragg wavelength of the particular Bragg grating lies between the central wavelengths of the detectors corresponding to maximum sensitivity. The detectors, depending on their central wavelengths, are also termed shortwave or longwave detectors. In a ground state of the FBG sensor, in which this is unstressed or uncompressed, for example, or exhibits a fixed reference temperature, the shortwave and the longwave detectors receive a certain reference energy component. If the lattice constant of the Bragg grating reduces, the shortwave and longwave detectors receive more or less energy respectively compared to the particular reference energy component. This process reverses if the lattice constant increases. Thus, with suitable calibration, the wavelength shift on the Bragg grating can be determined from the change in the detector signal. Depending on the number of Bragg gratings arranged in series or parallel in one or more FBG sensors, a large number of such narrowband spectrally responsive detector pairs can be combined, each being attuned to one of the Bragg gratings. The spectral detector sensitivity is preferably implemented by connecting detector elements to a narrowband transmission filter, eg interference filter.
The invention also concerns a strain measurement configuration using such a detector arrangement, and a method for operating it.